| Current MEMS microfilter improvement is driven by the need to perform absolute separation of micron-sized particles from milliliter-scale fluid volumes.; This research focuses on the problem of filter clogging. Our intent was to show that acoustic forces generated by a flexural plate wave (FPW) device act to reduce filter fouling. In an FPW-based assembly, acoustic agitation prevents particles from adhering to the filter element, while pumping sweeps away the particles that could potentially clog the filter. MEMS devices have already been shown to be effective for absolute filtration, but clogging has made them impractical for most applications. Also previously shown was the ability of FPW devices to move particles in a pumped liquid. Recognizing the FPW pumping as a self-cleaning mechanism for microfilters presents exciting possibilities for reduced filter fouling, allowing larger volumes of fluid to be filtered and extending filter lifetime.; A typical FPW device consists of a piezoelectric film (ZnO) on a 1-μm thick silicon nitride membrane supported by a silicon frame. The ZnO is sandwiched between a thin polysilicon ground plane and patterned aluminum fingers. This structure is referred to as an interdigitated transducers (IDT). When driven with a sinusoidal voltage, the IDT generates acoustic waves that propagate across the membrane. The new FPW-based microfilters have unidirectional IDTs across half of the membrane and pores in the other half, and are suitable for in-line filtering. Lithographically defined pores allow for a well-defined size threshold for particle removal, while acoustic forces act to reduce filter fouling.; Both lateral and vertical forces acting on a particle were identified and evaluated for typical operating conditions for particles of radii 0.1, 1.0, and 10 μm. Calculations show that the Stokes drag due to pressure-driven flow is the largest lateral force for all particle sizes. Van der Waals forces (for 0.1, and 1.0 μm spheres) and the Newtonian “added-mass” force (for 10 μm spheres) were found to be the largest vertical forces.; A stroboscopic laser interferometer developed by Dr. C. Rembe was used to measure dynamic membrane displacement for 800-μm wavelength FPW devices.; In the most important set of tests, FPW filters were allowed to clog, and then were agitated with acoustic waves to demonstrate clearing of pores. Acoustic particle manipulation was shown to free 2, 6, and 10 μm diameter spheres and sweep them away from once-blocked pores in both 4 μm and 8 μm filters. (Abstract shortened by UMI.)... |
